1. Field of the Invention
The present invention relates to a cellulose acylate film having an excellent transparency adapted for optical film and a process for producing the same, and to an optical film using the cellulose acylate film and an image display device.
2. Description of the Related Art
Cellulose acylate is used as a support for a photographic light-sensitive material due to its transparency, toughness and optical isotropy, and has an enlarging use as an optical film for use in image display devices including a liquid crystal display device and an organic EL display device. As an optical film for use in a liquid crystal display device, it is used as a protective film for a polarizing plate, or there has been practiced a method of stretching the film to generate in-plane retardation (Re) and retardation in the thickness direction (Rth) to prepare a retardation film for use in a liquid crystal display device of, for example, STN (Super Twisted Nematic) system.
In recent years, display elements of VA (Vertical Alignment) system and OCB (Optical Compensated Bend) system requiring a larger retardation of Re or Rth than STN system have been developed, and an optical film material excellent in generating an enough retardation is being required.
As a novel optical film material for meeting such requirement, there has been disclosed a solution cast film using a mixed ester of cellulose containing both acetyl group and propionyl group (cellulose acetate propionate)(JP-A-2001-188128). Also, since the melting point of cellulose acetate butyrate or cellulose acetate propionate is lower than that of cellulose acetate, there has been disclosed a method of using an optical film formed by melt-cast filming these cellulose acylates (JP-A-2000-352620). In comparison with the solution filming process, the melt-cast film production process has the advantages that it can eliminate the dissolving step and the drying step and that it imposes a less load on the environment.
As commercially available products of the cellulose acylates other than cellulose acetate, various cellulose acetate butyrate and cellulose acetate propionate have been disclosed for molding or painting use (Catalogue of Eastman Chemical Co. (1994)).